A tensioning device, such as a hydraulic tensioner, is used as a control device for a power transmission chain, or similar power transmission device, as the chain travels between a plurality of sprockets. In these devices, the chain transmits power from a driving sprocket to a driven sprocket. One portion of the chain span between the sprockets is tight or under tension while the other portion is slack. Generally, it is important to also impart and maintain a certain degree of tension in the slack portion of the chain to prevent noise, slippage, or the unmeshing of teeth in the case of a toothed chain.
Prevention of such slippage is particularly important in the case of a chain driven camshaft in an internal combustion engine because jumping of teeth will throw off the camshaft timing, possibly causing damage or rendering the engine inoperative. However, in the harsh environment of an internal combustion engine, various factors can cause fluctuations in the chain tension.
Wide variations in temperature and thermal expansion coefficients among the various parts of the engine can cause the chain tension to vary between excessively high or low levels. During prolonged use, wear to the components of the power transmission system can cause a decrease in chain tension. In addition, camshaft and crankshaft induced torsional vibrations may cause considerable variations in chain tensions. Reverse torque on an engine, occurring for example in stopping or in failed attempts at starting, can also cause fluctuations in chain tension. For these reasons, a mechanism is desired to remove excessive tensioning forces on the tight side of the chain and to ensure the necessary tension on the slack side of the chain.
Hydraulic tensioners are a common method of maintaining proper chain tension. In general, these mechanisms employ a lever arm that pushes against the chain on the slack side of the power transmission system. This lever arm must push toward the chain, tightening the chain, when the chain is slack, and must retract away from the chain when the chain tightens.
To accomplish this result, a hydraulic tensioner typically comprises a hollow piston or plunger, which is biased in the direction of the chain by a tensioner spring. The plunger is housed within a cylindrical bore in the tensioner body or housing, which has an interior space which is open at one end. A fluid chamber is formed in the interior space of the bore between the bore and the interior of the hollow piston. The fluid chamber receives fluid through a fluidic connection with a reservoir or exterior source of hydraulic fluid.
Typically, two types of valves are employed to regulate the flow of fluid into and out of the pressure chamber: inlet check valves and pressure relief valves. The inlet check valve typically includes a ball-check valve that opens to permit fluid flow through the valve and into the fluid chamber when the pressure in the chamber has decreased as a result of outward movement of the plunger. When the pressure in the pressure chamber is sufficiently high, the inlet check valve closes, preventing fluid from exiting the pressure chamber, which in turn prevents the piston from retracting, achieving a so-called xe2x80x9cno-returnxe2x80x9d function.
The pressure relief valve performs its function when the pressure in the chamber exceeds a predetermined maximum limit. The pressure relief valve opens to permit fluid to exit the chamber and allow the tensioner to retract in response to large increases in chain tension (and the associated fluid pressure). The pressure relief valve typically includes a spring biased check valve.
In operation, the inward force of the chain on the piston is balanced by the outward force of the tensioner spring and the reaction force from the hydraulic fluid. As the tension in the chain increases, the chain exerts an increased force on the plunger in the direction of plunger retraction. As the plunger is forced in the retraction direction, the fluid pressure in the pressure chamber increases, but the inlet check valve prevents the fluid from exiting the pressure chamber. If the pressure exceeds a predetermined maximum level, the pressure relief valve opens, allowing the fluid to exit the pressure chamber.
Examples of pressure relief valves are shown in the above-mentioned U.S. Pat. No. 5,577,970 and U.S. Pat. No. 5,07,309. In U.S. Pat. No. 5,577,970, the pressure relief valve is in the form of a reed valve that opens to permit fluid to exit the high pressure chamber. In U.S. Pat. No. 5,707,309, the pressure relief valve is integral with the inlet check valve.
According to one aspect of the present invention, there is provided a tensioner for providing a tensioning force to a power transmission chain that connects at least two rotating members such as a pair of sprockets. A hollow piston slidably fits within a bore in a piston housing, forming a fluid chamber. A spring is positioned within the fluid chamber to bias the piston outward from the bore. An inlet check valve permits flow from a source of pressurized fluid, or a reservoir, into the fluid chamber and prevents flow out of the chamber in the reverse direction.
A pressure relief valve is located in the nose of the piston. This valve permits fluid to exit the high pressure fluid chamber when the pressure in the pressure chamber reaches (or exceeds) a certain specified limit. The valve member includes a stem and rounded portion that closes against a seat member. A valve spring positioned between the seat and a retainer member biases the valve member into a closed position against the seat member. At a predetermined maximum pressure value, the resisting force of the valve spring is exceeded and the spring compresses to permit the valve member to move from the seat member. The seat member is either fixed or biased against the nose of the piston by a piston spring.
For a better understanding of these and other aspects and objects of the invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings.